Regulation of Humoral Immunity by Tox CD4 T cells are a critical component of the adaptive immune system. Through cell-to-cell contact and cytokine production they are required for the generation of effective antibody-mediated humoral immunity. As a link between environment and function, CD4 T helper cell differentiation is influenced by cytokines in the inflammatory milieu that in part shapes their capacity to produce lineage-specific cytokines and provide help to cells of the immune system. T follicular helper (TFH) cells are generated in the presence of IL-6 and IL-21 and are identified ex vivo by their expression of their master transcription factor BCL6 and cell surface expression of the chemokine receptor CXCR5 and PD-1. Based on their unique expression of CXCR5, TFH cells migrate to the B cell zone and provide help for antigen-bearing B cells. TFH cells are of critical importance for developing effective B cell responses as mice deficient in TFH-specific components (i.e. BCL6) fail to develop germinal centers or high affinity antibodies after infection or immunization. Although BCL6 is required for TFH differentiation, it is thought to directly bind and regulate a relatively small number of genes, suggesting that other downstream factors may be at play. In our preliminary data we show that Thymus-associated high mobility group box protein (Tox) is coordinately expressed with BCL6 and PD-1 in TFH cells and requires BCL6 for its expression. Tox is a DNA binding protein that has previously been shown to play a role in thymocyte development; however, its role in mature T cells is virtually unexplored. The goal of this application is to determine the role of Tox in the differentiation and/or function of TFH cell. Our hypothesis is that Tox promotes TFH development and that in the absence of Tox expression, T cells will not be able to support germinal center formation. We will test this hypothesis by manipulating Tox expression in mature T cells and examining the consequences of enhanced or reduced Tox expression levels by examining the magnitude and phenotype of TFH cells and the humoral response after infection and immunization. These studies will provide novel insight into the role of a previously under studied DNA binding protein in the differentiatio and function of TFH cells. Further, these studies will enhance our understanding of the transcription factor network that governs differentiation of this highly important subset of CD4 T cells.